The present invention generally relates to logic circuits, and more particularly to a logic circuit capable of carrying out a ternary (3-level) to binary (2-level) logic operation by use of a small number of active elements.
Presently, most logic circuits operate on binary logic with values of "0" and "1", and inverters, NOR circuits and NAND circuits are used as basic gates. Such basic gates are combined to achieve a desired function.
FIG. 1 shows an essential part of an example of the conventional exclusive-NOR circuit. The exclusive-NOR circuit comprises eight field effect transistors (FETs) Q1 through Q8, five resistors R1 through R5, two input terminals A and B, and an output terminal C. In FIG. 1, V.sub.DD denotes a positive power source voltage. This exclusive-NOR circuit requires eight active elements, that is, the eight FETs Q1 through Q8. However, in order to realize a logic circuit which can operate at a high speed, it is necessary to increase the operation speed of the basic gate or to make the basic gate have multiple functions so as to reduce the number of the basic gates which are required to realize the logic circuit.
As one candidate for the basic gate which will satisfy the above described demands, a resonant-tunneling hot electron transistor (hereinafter simply referred to as RHET) has been developed recently. FIG. 2 shows an essential part of an example of the conventional exclusive-NOR circuit employing the RHET. In FIG. 2, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. The exclusive-NOR circuit shown in FIG. 2 comprises an RHET Q9, resistors R7 through R9, the input terminals A and B, and the output terminal C.
The RHET Q9 has an approximately N-shaped gate voltage versus drain current characteristic, that is, a differential negative characteristic. Hence, this characteristic is effectively used in FIG. 2, and only one RHET is required as the basic gate to realize the exclusive-NOR circuit. The characteristic of the RHET may be effectively used to realize various other circuits, and the RHET is anticipated as one candidate for the basic gate which would realize a logic circuit capable of operating at a high speed, as discussed in "Resonant-Tunneling Hot Electron Transistors (RHET): Potential and Applications" by Naoki Yokoyama, The 18th (1986 International) Conference on Solid State Devices and Materials, 1986, pp. 347-350.
The exclusive-NOR circuit shown in FIG. 2 is superior compared to that shown in FIG. 1 in that the number of required elements is small and the operation speed is high. However, at the present, it is not easy to produce the RHET because of its vertical structure, and for this reason, it is difficult to produce an integrated circuit of the logic circuit employing the RHET.
Therefore, there is a demand to realize a logic circuit which employs the ordinary easily producible transistors but is capable of obtaining the superior effects obtainable when the RHET is used, such as the small number of required elements and the high operation speed. In other words, there is a demand to realize a logic circuit capable of carrying out a ternary-to-binary logic operation by use of the ordinary easily producible transistors.